There are fundamentally two possible chemical or biochemical approaches to attenuating the deleterious effects of ionizing radiation on organisms:                (1) attenuation of initial damage to biological structures, and        (2) improvement or acceleration of recovery.        
A number of compounds are known that provide some protection from ionizing radiation when they are present in the body during irradiation. Such compounds are typically antioxidants or free-radical scavengers that inactivate reactive chemical species formed during irradiation before they can damage important biological structures. Prominent examples of radioprotective compounds include cysteamine, 2-beta-amino-ethyl-isothiouronium-Br—HBr (AET), and S-2-(3-aminopropylamino)ethyl phosphorothioic acid (WR-2721). Since these compounds must be introduced into the organism before or during irradiation, they are obviously not useful in situations of unexpected or accidental exposure. Moreover, these compounds are toxic in humans.
The main possibilities for effective chemical therapy in organisms in which irradiation has already occurred are:                (1) to promote repair and recovery of individual cells within the organism, or        (2) to accelerate or enhance proliferation and/or differentiation of surviving stem cells.        
Bone marrow and intestinal epithelium are among the tissues most sensitive to radiation damage; attempts to promote recovery from irradiation need to focus on the stem cells in these tissues.
There exist several agents which can improve the survival of irradiated mammals when administered after irradiation. These include the yeast-derived polysaccharide Glucan, and polypeptide cytokines such as Interleukin-1, Granulocyte-Colony Stimulating Factor, and Granulocyte/Macrophage-Colony Stimulating Factor; all of these agents improve bone-marrow stem cell proliferation or differentiation. However, their efficacy is modest, producing Dose Reduction Factors less than 1.1 when administered after irradiation has already occurred, and their use is complicated by side effects. Moreover, they are all macromolecules which can only be administered parenterally.
There exists a need for compounds which effectively promote recovery when administered after exposure to ionizing radiation and which have important pharmaceutical qualities such as nontoxicity and activity after oral administration. Such agents would be useful in the cases of accidental exposure to ionizing radiation, and also in conjunction with radiation therapy for cancer, in order to promote recovery of normal tissue from irradiation. Such agents may also improve recovery from certain forms of chemical damage, e.g., bone-marrow suppression following either accidental or therapeutic exposure to compounds like cyclophosphamide or busulfan, which are both used in cancer chemotherapy.
It has been demonstrated that administration of exogenous deoxyribonucleic acid (DNA) to experimental animals after exposure to ionizing radiation can result in improved survival and functional recovery. Kanazir et al., Bull. Inst. Nuc. Sci. “Boris Kidrich” 9:145–153 (1959); Wilczok, T., et al., Int. J. Rad. Biol. 9:201–211 (1965); Golba, S., et al., Int. J. Rad. Biol. 13:261–268 (1967); U.S. Pat. No. 3,803,116.
Studies in cell cultures in vitro suggest that the actual restorative agents are deoxyribonucleosides, the enzymatic degradation products of DNA. Petrovic, D., et al., Int. J. Rad. Biol. 18:243–258 (1970). However, depolymerized DNA or deoxyribonucleosides administered to animals were ineffective in promoting survival or recovery after irradiation. Kanazir et al., Bull. Inst. Nuc. Sci. “Boris Kidrich” 9:145–153 (1959). There is reason to believe that this apparent contradiction is due to the rapid catabolism of deoxyribonucleosides in vivo by the enzymes in plasma and various organs. Thus, after administration of deoxyribonucleosides to rodents, tissues are exposed to effective concentrations for less than five minutes. Beltz et al., Bioch. Biophys., Acta. 297:258–267 (1973). In cell cultures, optimum survival after irradiation was found when deoxyribonucleosides were present in the culture medium for at least three hours. When DNA is administered parenterally, it is probably gradually depolymerized to give a sustained release of free deoxyribonucleosides into the circulation.
There may be other physiological or pathological conditions of mammalian tissue wherein the supply of exogenous deoxyribonucleosides may have therapeutic applications. Newman et al., Am. J. Physiol. 164:251–253 (1951), disclose a study in rats subjected to partial hepatectomy. The course of liver regeneration was followed for eleven days. The livers of rats treated with DNA regenerated significantly faster than did livers of untreated animals. It is likely that deoxyribonucleosides were the actual active agents in this study, since DNA is a large molecule that is not taken up efficiently by mammalian cells. Similarly, DNA applied to dermal wounds has been found to accelerate some aspects of the healing process, e.g., formation of granulation tissue. Dumont, Ann. Surg. 150:799–807 (1959); Marshak et al., Proc. Soc. Exp. Biol. Med. 58:62–63 (1945); Nicolau et al., Der Hautartzt 17:512–515 (1966). Yane and Kitano, U.S. Pat. No. 4,656,896, disclose evidence of beneficial effects of parenterally administered DNA in the treatment of gastric ulcers in rats.
In these examples, it is likely that the effect of DNA was related to its gradual degradation, resulting in the release of deoxyribonucleosides over a prolonged period. DNA is not, however, a suitable pharmaceutical agent to administer to humans, either orally or parenterally. In the case of oral administration, nucleosides released from DNA would mainly be degraded by enzymes in the intestinal lumen, in the intestinal walls, in plasma, and in the liver, rather than being available to tissues. Problems with parenterally administered DNA include possible antigenicity (exacerbated by adhering proteins which are difficult to remove during extraction), nonuniformity between batches, and possible undesirable effects not related to nucleoside release, e.g., enhancement of interferon re-lease from lymphocytes, which is a known effect of double-stranded nucleic acid.
The administration of deoxyribonucleosides has heretofore been contemplated for the reversal of obvious deficiencies of deoxyribonucleotides (e.g., thymidine administration to reverse toxicity caused by methotrexate, an antineoplastic agent which inhibits thymidine nucleotide biosynthesis; administration of deoxycytidine to reverse arabinosyl cytosine toxicity, or in people with deficiencies of particular enzymes (e.g., purine nucleoside phosphorylase) that ultimately result in impaired deoxyribonucleotide synthesis). Thymidine administration has also been considered as an antineoplastic treatment, since, in high concentrations, thymidine has cytostatic or cytotoxic properties.
However, the invention disclosed herein pertains to the recognition that unexpected beneficial effects may be obtained after administration of supraphysiological quantities of mixtures of deoxyribonucleosides in such a manner that they are available to tissues for a sustained period; this goal may be best accomplished through the use of the deoxyribonucleoside derivatives of the invention.